A METHOD OF FABRICATING A COMPOSITE MATERIAL PART BY SELF-PROPAGATING HIGH TEMPERATURE SYNTHESIS

Abstract

A method of fabricating a part made of ceramic matrix composite material, the method includes fabricating the part by forming a ceramic matrix in the pores of a fiber structure, the ceramic matrix being formed by self propagating high temperature synthesis from a powder composition present in the pores of the fiber structure,

Claims

1. A method of fabricating a part made of ceramic matrix composite material, the method comprising the following step: a) fabricating the part by forming a ceramic matrix in the pores of a fiber structure, the ceramic matrix being formed by self propagating high temperature synthesis from a powder composition present in the pores of the fiber structure; the matrix formed during step a) comprising a majority by weight: of Si.sub.2N.sub.2O formed by self propagating high temperature synthesis by chemical reaction between a silicon powder, a silica powder, and a gaseous phase comprising the element N; or of phases of TiN and of TiB.sub.2, these compounds being formed by self propagating high temperature synthesis by chemical reaction between a powder comprising titanium, a powder comprising boron, and a gaseous phase comprising the element N.

2. A method according to claim 1, wherein, prior to step a), a preliminary step b) is performed of densifying the fiber structure by a method other than the method of self propagating high temperature synthesis.

3. A method according to claim 1, wherein an additional step c) of densifying the part is performed after step a).

4.-8. (canceled)

9. A method according to claim 1, wherein the following steps are performed before step a): inserting at least a first powder into the pores of the fiber structure; and then inserting at least a second powder different from the first into the pores of the fiber structure; a ceramic matrix of composition that varies on going towards the outside surface of the part being obtained after step a).

10. A method according to claim 1, comprising a step of forming an environmental and/or thermal barrier, the environmental and/or thermal barrier being present after step a) over all or some of an outside surface of the part.

11.-12. (canceled)

13. A method according to claim 1, wherein the matrix formed during step a) comprises a majority by weight of Si.sub.2N.sub.2O formed by self propagating high temperature synthesis by chemical reaction between a silicon powder, a silica powder, and a gaseous phase comprising the element N, and wherein a powder comprising boron is present in the pores of the fiber structure prior to step a), and during step a) the powder comprising boron forms a BN phase by a nitriding reaction with the gaseous phase.

14. A part made of ceramic matrix composite material, the part comprising: a reinforcing fiber structure; and a ceramic matrix comprising a majority by weight of Si.sub.2N.sub.2O present in the pores of the fiber structure, the matrix presenting a content by weight of residual free silicon that is less than or equal to 5%.

15. (canceled)

16. A part according to claim 14, wherein the matrix comprises crystalline Si.sub.2N.sub.2O at a content by weight greater than or equal to 70%.

17. A turbine engine including a part according to claim 14.

18. A method of fabricating a part made of ceramic matrix composite material, the method comprising the following step: a) fabricating the part by forming a ceramic matrix in the pores of a fiber structure, the ceramic matrix being formed by self propagating high temperature synthesis from a powder composition present in the pores of the fiber structure; the matrix formed during step a) comprising a majority by weight: of TiC and of SiC, these compounds being formed by self propagating high temperature synthesis by chemical reaction between a powder comprising titanium, a powder comprising silicon, and a powder comprising carbon; or of AlN formed by self propagating high temperature synthesis by chemical reaction between a powder comprising aluminum, a carbon powder, and a gaseous phase comprising the element N.

19. A method according to claim 18, wherein, prior to step a), a preliminary step b) is performed of densifying the fiber structure by a method other than the method of self propagating high temperature synthesis.

20. A method according to claim 18, wherein an additional step c) of densifying the part is performed after step a).

21.-25. (canceled)

26. A method according to claim 18, wherein the following steps are performed before step a): inserting at least a first powder into the pores of the fiber structure; and then inserting at least a second powder different from the first into the pores of the fiber structure; a ceramic matrix of composition that varies on going towards the outside surface of the part being obtained after step a).

27. A method according to claim 10, further comprising a step of forming an environmental and/or thermal barrier, the environmental and/or thermal barrier being present after step a) over all or some of an outside surface of the part.

28.-29. (canceled)

30. A method of fabricating a part made of ceramic matrix composite material, the method comprising the following step: a) fabricating the part by forming a ceramic matrix in the pores of a fiber structure, the ceramic matrix being formed by self propagating high temperature synthesis from a powder composition present in the pores of the fiber structure; the matrix formed during step a) comprising a majority by weight: of phases of BN and of TiCN, these compounds being formed by self propagating high temperature synthesis by chemical reaction between a powder comprising titanium, a powder comprising boron and carbon, and a gaseous phase comprising the element N; or of phases of Al.sub.2O.sub.3 and of SiC, these compounds being formed by self propagating high temperature synthesis by chemical reaction between a powder comprising silicon and oxygen, a powder comprising aluminum, and a powder comprising carbon; or of a SiAlON type compound formed by self propagating high temperature synthesis by chemical reaction between a silicon powder, a silica powder, a powder comprising aluminum, and a gaseous phase comprising the element N.

31. A method according to claim 30, wherein, prior to step a), a preliminary step b) is performed of densifying the fiber structure by a method other than the method of self propagating high temperature synthesis.

32. A method according to claim 30, wherein an additional step c) of densifying the part is performed after step a).

33.-37. (canceled)

38. A method according to claim 30, wherein the following steps are performed before step a): inserting at least a first powder into the pores of the fiber structure; and then inserting at least a second powder different from the first into the pores of the fiber structure; a ceramic matrix of composition that varies on going towards the outside surface of the part being obtained after step a).

39. A method according to claim 30, further comprising a step of forming an environmental and/or thermal barrier, the environmental and/or thermal barrier being present after step a) over all or some of an outside surface of the part.

40.-41. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0109] Other characteristics and advantages of the invention appear from the following description of particular implementations of the invention, given as nonlimiting examples, and with reference to the accompanying drawings, in which:

[0110] FIG. 1 is a flowchart of an example of a method of the invention;

[0111] FIGS. 2 and 3 are flowcharts of variant methods of the invention;

[0112] FIGS. 4 to 6 are more detailed flowcharts of examples of methods of the invention;

[0113] FIGS. 7 and 8 are photographs obtained by a scanning electron microscope of a composite material part formed by performing an implementation of the method of the invention; and

[0114] FIG. 9 is a photograph obtained by a scanning electron microscope of a part made of composite material formed by performing another implementation of the method of the invention.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

[0115] FIG. 1 shows a succession of steps of a method of the invention. A powder composition is initially prepared (step 10). The powder composition may comprise a mixture of an Si powder and an SiO.sub.2 powder when it is desired to obtain a matrix based on Si.sub.2N.sub.2O.

[0116] Thereafter, the mixture prepared in step 10 is put into suspension in a liquid medium, e.g. in water (step 20). When it is desired to obtain a matrix based on Si.sub.2N.sub.2O, the silicon and silica powders may be present in the liquid medium at a volume content (sum of the volume content of the silicon powder plus the volume content of the silica powder) that is greater than or equal to 15%, e.g. lying in the range 15% to 25%.

[0117] The powder composition in suspension in the liquid medium is then introduced into the pores of a fiber structure, e.g. by a submicron powder aspiration (SPA) type method (step 30).

[0118] Synthesis is then performed by self-sustaining reaction at high temperature, enabling a matrix to be formed in the pores of the fiber structure (step 40). Throughout all or some of step a), the fiber structure may be present in a volume that is maintained at a temperature less than or equal to 1500 C., e.g. less than or equal to 1450 C.

[0119] FIG. 2 is a flowchart showing a variant implementation of a method of the invention.

[0120] In a first step 31, a precursor composition is introduced into the pores of a fiber structure. By way of example, the precursor composition may comprise a polymer that is to be pyrolyzed in order to form the powder composition. In a variant, the precursor composition is in the form of a powder, e.g. a silicon powder when it is desired to obtain a matrix based on Si.sub.2N.sub.2O.

[0121] Thereafter, the precursor composition is transformed into a powder composition (step 35). For example, when the precursor composition comprises a silicon powder, it is possible to perform oxidation treatment on the silicon powder in order to obtain a powder composition comprising silica powder and silicon powder so as to form a matrix based on Si.sub.2N.sub.2O by a self propagating high temperature synthesis reaction. By way of example, the heat treatment performed for transforming a portion of the silicon powder into silica may include subjecting the silicon powder to a temperature of 900 C. for 1 h in air. In a variant, when the precursor composition includes a polymer, step 35 may include heat treatment for pyrolyzing said polymer in order to obtain the powder composition.

[0122] Synthesis by a self propagating high temperature reaction enables a matrix to be formed in the pores of the fiber structure (step 40).

[0123] When it is desired to obtain a matrix of Si.sub.2N.sub.2O, the fact of using a precursor composition in the form of a silicon powder and of transforming it in part into silica directly in the fiber structure makes it possible to obtain better densification. This transformation makes it possible to obtain better filling of the pores of the fiber structure after partial oxidation of the silicon powder. For example, if a silicon powder is inserted by SPA so as to fill the pores of the fiber structure by 55%, it is possible, after oxidation, to obtain filling of the pores of the fiber structure of 68%. By forming silica in situ, it is thus possible after step a) to obtain a matrix presenting particularly little residual porosity.

[0124] FIG. 3 is a flowchart showing a variant implementation of a method of the invention. During step 32, a first powder mixture is inserted into the pores of the fiber structure. Thereafter, a second powder mixture, different from the first powder mixture, is inserted into the fiber structure (step 33). Because of the prior introduction of the first powder mixture, the quantity of the second powder mixture present in the fiber structure reduces with increasing depth into the fiber structure. As a result of this variation, it becomes possible after step 41 to obtain a ceramic matrix of composition that varies on going towards an outside surface of the part. Under such circumstances, after step a), it is advantageously possible to form an environmental and/or thermal barrier defining all or some of the outside surface of the part. Consequently, such a variant method presents the advantage of making it possible in a single step both to densify the fiber structure and to form an environmental and/or thermal barrier.

[0125] With reference to FIGS. 4 to 6, there follows a description of a few examples of successions of steps that can be performed in the context of the invention when it is desired to obtain a matrix based on Si.sub.2N.sub.2O. For reasons of concision, these figures relate only to forming a matrix based on Si.sub.2N.sub.2O, however, and as explained above, it should naturally be understood that the invention is not limited to forming matrices based on Si.sub.2N.sub.2O.

[0126] FIG. 4 shows the succession of steps of an example method of the invention. During step 2, a plurality of yarns are transformed into a fiber structure, e.g. a 2D or a 3D structure. This may be done using any method known to the person skilled in the art.

[0127] The yarns 1 that are used comprise a plurality of ceramic and/or carbon fibers. By way of example, the ceramic fibers are SiC fibers. By way of example, suitable SiC fibers are supplied under the names Nicalon, Hi-Nicalon or Hi-Nicalon-S by the Japanese supplier NGS, or under the name Tyranno SA3 by the supplier UBE. By way of example, suitable carbon fibers are supplied under the name Torayca by the supplier Toray.

[0128] Thereafter, an interphase is made on the yarns (step 4). The interface serves advantageously to increase the mechanical strength of the ceramic matrix composite material, in particular by deflecting any cracks in the matrix so that they do not affect the integrity of the fibers. In a variant, the fibers are not coated in an interphase. Thereafter, it is then possible to perform annealing treatment (step 6, which is optional in the example method being described).

[0129] During step 8, a consolidation coating is formed on the fibers. For this purpose, the fiber structure may be placed in a shaper and the consolidation coating may be formed by chemical vapor infiltration. By way of example, the consolidation coating comprises a carbide, with the consolidation coating being SiC, B.sub.4C, and/or SiBC, for example. The consolidation coating may constitute a chemical and/or thermal barrier serving to protect the fibers and the interphase (if there is one) from possible degradation.

[0130] It is then possible to perform a step of machining the fiber structure (step 12). After this machining step, the powder composition present in the form of a suspension in a liquid medium is inserted into the fiber structure by a sub-micrometer powder suction method (step 30). Thereafter, self propagating high temperature synthesis is performed in order to perform the matrix based on Si.sub.2N.sub.2O (step 40).

[0131] As mentioned above, Si.sub.2N.sub.2O presents numerous advantages. In particular, this material presents greater resistance to oxidation than does SiC (oxidation start temperature: 1600 C. under dry air). Furthermore, the mechanical properties of the Si.sub.2N.sub.2O material are compatible with it being associated in the part being made both with the SiC fibers (e.g. of the Hi-Nicalon S type) and also with an Si.sub.3N.sub.4 phase (see table 1 below).

TABLE-US-00001 Si.sub.2N.sub.2O Si.sub.3N.sub.4 SiC Young's modulus 230 320 400 (GPa) Specific gravity 2.81 3.27 3.20 Expansion 3.5 10.sup.6 3.6 10.sup.6 5 10.sup.6 coefficient (K.sup.1)

[0132] When the matrix formed during step a) has a majority phase of Si.sub.2N.sub.2O, it is possible to form compounds other than Si.sub.2N.sub.2O, such as Si.sub.3N.sub.4, in the matrix. The content by weight of Si.sub.3N.sub.4 in the matrix formed during step a) is less than 50%, preferably less than or equal to 5%.

[0133] There follow a few examples of structures that may be formed by a method of the invention: [0134] SiC/Interphase, PyC/SiC consolidation, or Si.sub.3N.sub.4/matrix fibers including Si.sub.2N.sub.2O formed by self propagating high temperature synthesis from a mixture of Si and SiO.sub.2 powders under nitrogen pressure; [0135] SiC/Interphase, PyC/SiC consolidation, or Si.sub.3N.sub.4/matrix fibers including Si.sub.2N.sub.2O and a compound of formula Si.sub.xB.sub.y formed by self propagating high temperature synthesis from a mixture of Si, SiO.sub.2, and B powders under nitrogen. pressure; [0136] SiC/Interphase, PyC/SiC consolidation, or Si.sub.3N.sub.4/matrix fibers including Si.sub.2N.sub.2O and SiAlON formed by self propagating high temperature synthesis from a mixture of Si, SiO.sub.2, and Al powders under nitrogen pressure.

[0137] FIG. 5 shows a variant method of the invention. The sequence of steps in FIG. 5 differs from that in FIG. 4 by the method used for introducing the powder composition into the fiber structure and by the fact that post-treatment is performed after step a).

[0138] In the example of FIG. 5, the powder composition in the form of an aqueous suspension is inserted into the fiber matrix by injection, with heat treatment then being performed to evaporate the water. The injection and the heat treatment that are performed may be of the same type as those implemented in methods of molding by injecting resin (methods known as resin transfer molding or RTM). It is possible to use other methods known to the person skilled in the art to insert the powder composition into the fiber structure, and thus, by way of example, it is possible to insert the powder composition into the fiber structure by electrophoresis.

[0139] Once the self propagating high temperature synthesis has been performed, an additional densification step c) may be performed, e.g. by cycles of impregnating and pyrolyzing a polymer in order to fill in the residual porosity of the resulting matrix (step 50). In a variant, during step 50, an additional step of spark plasma sintering (SPS) densification may be performed in order to increase the final density of the part.

[0140] FIG. 6 shows the succession of steps in another example method of the invention. In this example, an interphase is initially formed on the yarns prior to forming the fiber structure (step 3). Thereafter, the yarns are subjected to sizing and/or wrapping treatment followed by a textile operation such as weaving in order to obtain the fiber structure (step 5). The powder composition is inserted into the fiber structure, a matrix is formed by self propagating high temperature synthesis, and then additional densification is performed, as explained above.

EXAMPLES

Example 1

Forming a Matrix Having an Si.SUB.2.N.SUB.2.O Phase From Silicon and Silica Powders

[0141] The sequence of the various steps of a method of the invention is described below.

[0142] Commercially-available powders of silicon and of silica were initially subjected to attrition grinding. Silica and silicon powders made available by the supplier Sigma-Aldrich were used. Before grinding, the silica powder used presented grains having a median diameter (D50) equal to 2.1 m, and the silicon powder used presented grains having a median diameter (D50) equal to 11 m.

[0143] Grinding served to adjust the grain sizes of the silica and silicon powders. After grinding, the median diameter (D50) of the grains of silica powder was about 600 nm and the median diameter (D50) of the grains of silicon powder was about 400 nm.

[0144] The silicon powder was then subjected to heat treatment at 600 C. for 6 h in air in order to improve its wettability.

[0145] A stable aqueous suspension filled with the powders up to 20% by volume was then prepared. The suspension presented a pH lying in the range 9 to 9.5 and it was stabilized by adding tetramethylammonium hydroxide (TMAH). The Si/SiO.sub.2 molar ratio in the suspension was about 3.

[0146] A fiber preform was impregnated by a submicron powder aspiration (SPA) method under a pressure of 4 bars and with application of a vacuum for 2 h. The fiber preform used had a plurality of SiC fibers sold under the name Hi-Nicalon-S, coated with a PyC interface having a thickness of 100 nm and an SiC consolidation coating having a thickness of 1 m. The fiber preform used presented initial porosity of 54% (results obtained by three different measurements giving similar values: helium pycnometry, water impregnation, mercury porosimetry).

[0147] Once the powders had been inserted in the fiber preform, the following heat treatment under pressure was performed to implement self propagating high temperature synthesis (SHS method): [0148] raise temperature at 250 C./min up to a temperature of 1400 C. by Joule effect heating; [0149] pause for 30 min at 1400 C. under a pressure of 20 bars of nitrogen; and [0150] controlled cooling down to ambient temperature.

[0151] After performing such a protocol, the results given in FIGS. 7 and 8 were obtained. The matrix achieved effective and uniform filling of the pores in the fiber structure. The fibers, the interface, and the consolidation coating were not damaged by the self propagating high temperature synthesis.

[0152] After XRD analysis, the following results were obtained for the composition of the matrix: 75% by weight of crystalline Si.sub.2N.sub.2O, presence of phases of -Si.sub.3N.sub.4 and -Si.sub.3N.sub.4, and concentration of residual free silicon in the matrix less than or equal to 5% by weight. The residual porosity of the resulting material was about 23% (measured by immersion in water).

[0153] The step of impregnating the preform by SPA followed by self propagating high temperature synthesis made it possible to fill in about 55% of the initial porosity.

Example 2

Forming a Matrix Comprising an Si.SUB.2.N.SUB.2.O Phase Involving Partial Oxidation Pre-Treatment Prior to Step a)

[0154] The sequence of the various steps of another example method of the invention is described below:

[0155] 1/ Grinding SiO.sub.2 powder under water and Si powder under isopropanol/ethanol, The powders presented a median diameter lying in the range 0.5 m to 1 m.

[0156] 2/ Subjecting the silicon powder to heat treatment at 600 C. for 6 h under air.

[0157] 3/ obtaining a powder mixture constituted by 73% by weight of Si and 27% by weight of SiO.sub.2, giving an Si/SiO.sub.2 molar ratio of 5.77.

[0158] 4/ Putting the powders into suspension in water, the suspension being stabilized at a pH of 9 with TMAH, and presenting a dry matter content of 20% by volume.

[0159] 5/ Impregnating by SPA (4 bar to vacuum for 2 h) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0160] 6/ Drying the material at 100 C. overnight.

[0161] 7/ Oxidation heat treatment of the silicon powder after the SPA step: 1 h at 900 C. under air.

[0162] 8/ Heat treatment by self propagating high temperature synthesis: rise to 1450 C. at a rate of 200 C./min under 20 bar of dinitrogen. Pause at temperature for 10 min. Controlled cooling down to ambient temperature.

[0163] The matrix was made up of 86% by weight of crystalline Si.sub.2N.sub.2O, 12% by weight of Si.sub.3N.sub.4, and 2% by weight of Si.

[0164] The residual porosity was about 17%.

[0165] The matrix was uniform and dense (see FIG. 9) and the residual porosity of the material obtained was even less than that of material obtained in example 1.

Example 3

Forming a Matrix Having an Si.SUB.2.N.SUB.2.O Phase From Silicon, Silica, and Boron Powders

[0166] The sequence of the various steps of another example method of the invention is described below:

[0167] 1/ Separate grinding of the Si and B powders under isopropanol/ethanol, and of the SiO.sub.2 in distilled water.

[0168] 2/ Subjecting the silicon powder to heat treatment at 600 C. in air in order to facilitate putting it into suspension.

[0169] 3/ Mixing together the powders having the following composition by weight: Si/SiO.sub.2=1.4+10% B.

[0170] 4/ Putting into aqueous suspension (dry matter content lying in the range 15% to 20% by volume). pH was controlled by adding a strong base, TMAH.

[0171] 5/ Impregnating by SPA in a fiber preform.

[0172] 6/ Drying.

[0173] 7/ Heat treatment by the method of self propagating high temperature synthesis: rise to 1450 C. at a rate of 200 C./min under 20 bar or 30 bar of dinitrogen. Pause at temperature for 10 min. Controlled cooling down to ambient temperature.

Example 4

Forming a Matrix Comprising Phases of TiN and TiB.SUB.2

[0174] The sequence of the various steps of another example method of the invention is described below:

[0175] 1/ Mixing commercially-available powders of Ti and BN with the following composition: BN/Ti=1 (molar ratio) i.e. BN/Ti=0.52 (weight ratio). The grains of the powders used had a median diameter lying in the range 0.5 m to 1 m.

[0176] 2/ Putting the powders into suspension in ethanol and adding 2 milligrams per square meter (mg/m.sup.2) of polyethylene imine (PEI) as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0177] 3/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0178] 4/ Drying.

[0179] 5/ Heat treatment by the method of self propagating high temperature synthesis: rise to 950 C. at a rate of 200 C./min under 40 bar of dinitrogen. Controlled cooling down to ambient temperature.

[0180] BN, Ti and N.sub.2 react within the pores of the fiber structure in order to obtain a matrix based on TiB.sub.2 and on TiN, with the following reaction:

yTi.sub.(s)+xBN.sub.(s)+0.5(y1.5x)N.sub.2(g).fwdarw.0.5xTiB.sub.2(s)+(y0.5x)TiN.sub.(s)

[0181] This method obtained a matrix comprising 55% by weight of TiB.sub.2 and 35% by weight of TiN. The maximum combustion temperature remained less than 1500 C.

Example 5

Forming a Matrix Comprising Phases of TiC and SiC

[0182] The sequence of the various steps of another example method of the invention is described below:

[0183] 1/ Grinding the silicon powder in isopropanol/ethanol. The powder had a median diameter lying in the range 0.5 m to 1 m.

[0184] 2/ Mixing the powders in order to obtain a mixture constituted by 48% by weight of Ti (commercially-available titanium powder), 28% by weight of Si, and 24% by weight of C (0.8 m Luvomax).

[0185] 3/ Putting the powders into suspension in ethanol and adding 2 mg/m.sup.2 of PEI as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0186] 4/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0187] 5/ Heat treatment by the method of self propagating high temperature synthesis: initiating the reaction at 650 C. under an inert atmosphere. Controlled cooling down to ambient temperature.

[0188] The Ti, Si, and C powders react inside the pores of the fiber structure in order to obtain a matrix made up of TiC and SiC, with the following reaction:

xTi.sub.(s)+(1x)Si.sub.(s)+C.sub.(s).fwdarw.xTiC.sub.(s)+(1x)SiC.sub.(s)

[0189] The reaction that leads to TiC being synthesized by self propagating high temperature synthesis is extremely exothermic and fast. In contrast, although the reaction between Si and C for forming SiC is also exothermic, it is not sufficiently exothermic to be self propagating.

[0190] Thus, a material made up of TiC and SiC can be synthesized by coupling a powerful exothermic reaction (Ti+C) with one that is less so (Si+C). It should be observed that under such circumstances, the reaction does not require the participation of a gaseous phase in order to propagate.

Example 6

Forming a Matrix Comprising a Phase of AlN

[0191] The sequence of the various steps of another example method of the invention is described below:

[0192] 1/ Mixing together commercially-available powders having a weight ratio of Al/AlN=1 or of Al/Si.sub.3N.sub.4=1, with NH.sub.4F being present in the mixture at 3% by weight and C being present in the mixture at 3% by weight.

[0193] 2/ Putting the powders into suspension in ethanol and adding 2 mg/m.sup.2 of PEI as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0194] 3/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0195] 4/ Heat treatment by self propagating high temperature synthesis: initiating at 1100 C. with heating at a rate of 200 C./min under 50 bar of dinitrogen. Pause at temperature for 30 min. Controlled cooling down to ambient temperature.

[0196] The aluminum reacts with the gaseous phase to form AlN in the pores of the fiber preform, with the following reaction:

2Al.sub.(s)+N.sub.2(g).fwdarw.2AlN.sub.(s)

[0197] Adding carbon powder serves advantageously to increase the nitriding yield of the aluminum by reacting with the protective layer of alumina on the particles of aluminum (chemical reduction).

Example 7

Forming a Matrix Comprising Phases of BN and of TiCN

[0198] The sequence of the various steps of another example method of the invention is described below:

[0199] 1/ If necessary, grinding the B.sub.4C in isopropanol/ethanol.

[0200] 2/ Mixing together the powders with a molar ratio Ti/B.sub.4C=1.

[0201] 3/ Putting the powders into suspension in ethanol and adding 2 mg/m.sup.2 of PEI as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0202] 4/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0203] 5/ Heat treatment by self propagating high temperature synthesis: initiating the reaction by an electrical filament, under 1000 bar of dinitrogen.

[0204] The powders react with the gaseous phase to form a matrix made up of BN and TiCN in the pores of the fiber structure, with the following reaction:

xTi.sub.(s)+B.sub.4C.sub.(s)+[(4+y)/2]N.sub.2(g).fwdarw.4BN.sub.(s)+Ti.sub.xCN.sub.y(s)

[0205] TiN is preferably formed during the combustion stage. Free carbon coming from the decomposition of B.sub.4C diffuses in the TiN lattice to form a TiCN phase.

Example 8

Forming a Matrix Comprising Phases of Al.SUB.2.O.SUB.3 .and SiC

[0206] The sequence of the various steps of another example method of the invention is described below:

[0207] 1/ Grinding (SiO.sub.2)a quartz in distilled water. The powders obtained present a median diameter lying in the range 0.5 m to 1 m.

[0208] 2/ Mixing together SiO.sub.2, commercial Al, and commercial C powders in respective proportions by weight of 56%, 33%, and 11%.

[0209] 3/ Putting the powders into suspension in ethanol and adding 2 mg/m.sup.2 of PEI as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0210] 4/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0211] 5/ Heat treatment by self propagating high temperature synthesis: initiating the reaction by an electrical filament, under an inert atmosphere.

[0212] The powders react in the pores to form SiC and Al.sub.2O.sub.3, with the following reaction:

3SiO.sub.2(s)+4Al.sub.(s)+3C.sub.(s).fwdarw.3SiC.sub.(s)+2Al.sub.2O.sub.3(s)

Example 9

Forming a Matrix Comprising a Phase of SiAlON Type

[0213] The sequence of the various steps of another example method of the invention is described below:

[0214] 1/Grinding SiO.sub.2 powder under water and Si powder under isopropanol/ethanol. The powders presented a median diameter lying in the range 0.5 m to 1 m.

[0215] 2/ Mixing together the powders with the following composition by weight: 70% Si, 15% SiO.sub.2, and 15% commercial Al. A small amount of Y.sub.2O.sub.3 was added (about 3% by weight) to further stabilise the resulting SiAlON in phase.

[0216] 3/ Putting the powders into suspension in ethanol and adding 2 mg/m.sup.2 of PEI as steric dispersant. The quantity of dry matter in suspension lay in range 15% to 20% by volume.

[0217] 4/ Impregnating by SPA (4 bar to vacuum) into a fiber structure made up of ceramic fibers coated with an interphase of pyrocarbon and SiC consolidation.

[0218] 5/ Heat treatment by self propagating high temperature synthesis: rise to 1400 C. at a rate of 200 C./min under 10 bar of dinitrogen. Pause at temperature for 30 min. Controlled cooling down to ambient temperature.

[0219] SiAlON is formed in the pores of the preform by the following reaction:

4.5Si.sub.(s)+Al.sub.(s)+0.5SiO.sub.2(s)+3.5N.sub.2(g).fwdarw.-Si.sub.5AlON.sub.7(s)

Example 10

Forming a Matrix Comprising a Phase of Si.SUB.2.N.SUB.2.O and an Environmental/Thermal Barrier

[0220] The sequence of the various steps of another example method of the invention is described below:

[0221] 1/ Grinding powders of SiO.sub.2, of mullite (3Al.sub.2O.sub.3.2SiO.sub.2), and of Y.sub.2Si.sub.2O.sub.7 under water, and of Si under isopropanol/ethanol. The powders presented a median diameter lying in the range 0.5 m to 1 m.

[0222] 2/ First impregnating by SPA of a fiber structure with a first powder mixture, constituted by 58% by weight of Si and 42% by weight of SiO.sub.2, previously put into aqueous suspension at pH 9, the suspension being filled at 20% by volume.

[0223] 3/ Drying the material at 100 C. overnight.

[0224] 4/ Second impregnation of the fiber structure by vacuum transfer of a second powder mixture constituted by 72% by weight of Y.sub.2Si.sub.2O.sub.7 and by 28% by weight of mullite, previously put into aqueous suspension at pH 12, the suspension being filled at 25% by volume.

[0225] 5/ Drying the material at 100 C. overnight.

[0226] 6/ Heat treatment by self propagating high temperature synthesis: rise to 1450 C. at a rate of 200 C./min under 20 bar of dinitrogen. Pause at temperature for 30 min. Controlled cooling down to ambient temperature.

[0227] Si.sub.2N.sub.2O is formed in the pores of the fiber structure, and Al.sub.2O.sub.3, Y.sub.2O.sub.3, and SiO.sub.2 are formed on the surface of the part, over a depth of a few tens of micrometers.

[0228] The term comprising/containing a should be understood as comprising/containing at least one.

[0229] The term lying in the range . . . to . . . should be understood as including the limits.